Rust Intro

Mozilla
The language grew out of a personal project by Mozilla employee Graydon Hoare.
Mozilla began sponsoring the project in 2009 and announced it in 2010. The same
year, work shifted from the initial compiler (written in OCaml) to the self-hosting
compiler written in Rust. Known as rustc, it successfully compiled itself in 2011.
rustc uses LLVM as its back end.

Safe, concurrent, practical language
Rust is a general-purpose, multi-paradigm, compiled programming language.
It is designed to be a "safe, concurrent, practical language", supporting pure-
functional, imperative-procedural, and object-oriented styles.

Object oriented
Structs
Enums
Method Syntax
Generics
Traits

Structs
struct Point {
x: i32,
y: i32,
}
fn main() {
let origin = Point { x: 0, y: 0 }; // origin: Point
println!("The origin is at ({}, {})", origin.x, origin.y);
}

Enums
enum BoardGameTurn {
Move { squares: i32 },
Pass,
}
let y: BoardGameTurn = BoardGameTurn::Move { squares: 1 };

Generics
struct Point<T> {
x: T,
y: T,
}
let int_origin = Point { x: 0, y: 0 };
let float_origin = Point { x: 0.0, y: 0.0 };

Method syntax
struct Circle {
x: f64,
y: f64,
radius: f64,
}
fn main() {
let c = Circle { x: 0.0, y: 0.0, radius: 2.0 };
println!("{}", c.area());
}

Method syntax
impl Circle {
fn area(&self) -> f64 {
std::f64::consts::PI * (self.radius * self.radius)
}
}

Traits
trait HasArea {
fn area(&self) -> f64;
}
impl HasArea for Circle {
fn area(&self) -> f64 {
std::f64::consts::PI * (self.radius * self.radius)
}
}

Traits
fn print_area<T: HasArea>(shape: T) {
println!("This shape has an area of {}", shape.area());
}

Functional
Functions
Function pointers
Type inference
Immutable by default
Option and Result
Pattern matching
Lambda functions

Functions
fn add_one(x: i32) -> i32 {
x + 1
}

Functions
x + 1;
}
We would get an error:
error: not all control paths return a value
x + 1;
}
help: consider removing this semicolon:
x + 1;
^

Functions
fn plus_one(i: i32) -> i32 {
i + 1
}
let f: fn(i32) -> i32 = plus_one;
let six = f(5);

Type inference
Rust has this thing called ‘type inference’. If it can figure out what the type of
something is, Rust doesn’t require you to actually type it out.
let x = 5; // x: i32
fn plus_one(i: i32) -> i32 {
i + 1
}
// without type inference
let f: fn(i32) -> i32 = plus_one;
// with type inference
let f = plus_one;

Mutability
let x = 5;
x = 10;
error: re-assignment of immutable variable `x`
x = 10;
^~~~~~~

Mutability
let mut x = 5; // mut x: i32
x = 10;

Option
pub enum Option<T> {
None,
Some(T),
}

Option
fn divide(numerator: f64, denominator: f64) -> Option<f64> {
if denominator == 0.0 {
None
} else {
Some(numerator / denominator)
}
}

Result
enum Result<T, E> {
Ok(T),
Err(E)
}

fn divide(numerator: f64, denominator: f64) -> Result<f64, &'static str> {
if denominator == 0.0 {
Err("Can not divide by zero")
} else {
Ok(numerator / denominator)
}
}
Result

Pattern matching
let x = 5;
match x {
1 => println!("one"),
2 => println!("two"),
3 => println!("three"),
4 => println!("four"),
5 => println!("five"),
_ => println!("something else"),
}

Pattern matching
One of the many advantages of match is it enforces ‘exhaustiveness checking’.
For example if we remove the last arm with the underscore _, the compiler will
give us an error:
error: non-exhaustive patterns: `_` not covered

Pattern matching
// The return value of the function is an option
let result = divide(2.0, 3.0);
// Pattern match to retrieve the value
match result {
// The division was valid
Some(x) => println!("Result: {}", x),
// The division was invalid
None => println!("Cannot divide by 0"),
}

Pattern matching
// The return value of the function is an Result<f64, &'static str>
let result = divide(2.0, 3.0);
// Pattern match to retrieve the value
match result {
// The division was valid
Ok(x) => println!("Result: {}", x),
// The division was invalid
Err(e) => println!(e),
}

Pattern matching
Again, the Rust compiler checks exhaustiveness, so it demands that you have a
match arm for every variant of the enum. If you leave one off, it will give you a
compile-time error unless you use _ or provide all possible arms.

Lambda functions
let plus_one = |x: i32| x + 1;
assert_eq!(2, plus_one(1));

Closures
let num = 5;
let plus_num = |x: i32| x + num;
assert_eq!(10, plus_num(5));

Function pointers
fn call_with_one(some_closure: &Fn(i32) -> i32) -> i32 {
some_closure(1)
}
fn add_one(i: i32) -> i32 {
i + 1
}
let f = add_one;
let answer = call_with_one(&f);
assert_eq!(2, answer);

Memory Safe
The Stack and the Heap
Ownership
Borrowing
Lifetimes

The Stack
fn foo() {
let y = 5;
let z = 100;
}
fn main() {
let x = 42;
foo();
}

The Stack
fn foo() {
let y = 5;
let z = 100;
}
fn main() {
let x = 42; <-
foo();
}
Address Name Value
0 x 42

The Stack
fn foo() {
let y = 5;
let z = 100;
}
fn main() {
let x = 42;
foo(); <-
}
Address Name Value
2 z 1000
1 y 5
0 x 42

The Stack
fn foo() {
let y = 5;
let z = 100;
}
fn main() {
let x = 42;
foo();
} <-
Address Name Value
0 x 42

The Heap
fn main() {
let x = Box::new(5);
let y = 42;
}
Address Name Value
(230) - 1 5
... ... ...
1 y 42
0 x → (230) - 1

Ownership
let v = vec![1, 2, 3];
let v2 = v;
println!("v[0] is: {}", v[0]);
error: use of moved value: `v`
println!("v[0] is: {}", v[0]);
^

Ownership
fn take(v: Vec<i32>) {
// what happens here isn’t important.
}
let v = vec![1, 2, 3];
take(v);
println!("v[0] is: {}", v[0]);
Same error: ‘use of moved value’.

Borrowing
fn foo(v1: Vec<i32>, v2: Vec<i32>) -> (Vec<i32>, Vec<i32>, i32) {
// do stuff with v1 and v2
// hand back ownership, and the result of our function
(v1, v2, 42)
}
let v1 = vec![1, 2, 3];
let v2 = vec![1, 2, 3];
let (v1, v2, answer) = foo(v1, v2);

Borrowing
fn foo(v1: &Vec<i32>, v2: &Vec<i32>) -> i32 {
// do stuff with v1 and v2
// return the answer
42
}
let v1 = vec![1, 2, 3];
let v2 = vec![1, 2, 3];
let answer = foo(&v1, &v2);
// we can use v1 and v2 here!

&mut references
fn foo(v: &Vec<i32>) {
v.push(5);
}
let v = vec![];
foo(&v);
errors with:
error: cannot borrow immutable borrowed content `*v` as mutable
v.push(5);
^

&mut references
fn foo(v: &mut Vec<i32>) {
v.push(5);
}
let mut v = vec![];
foo(&mut v);

The Rules
First, any borrow must last for a scope no greater than that of the owner. Second,
you may have one or the other of these two kinds of borrows, but not both at the
same time:
one or more references (&T) to a resource,
exactly one mutable reference (&mut T).

Rust prevents data races at compile time
There is a ‘data race’ when two or more pointers access the same memory
location at the same time, where at least one of them is writing, and the operations
are not synchronized.

Lifetimes
1. I acquire a handle to some kind of resource.
2. I lend you a reference to the resource.
3. I decide I’m done with the resource, and deallocate it, while you still have your
reference.
4. You decide to use the resource.

Lifetimes
// implicit
fn foo(x: &i32) {
}
// explicit
fn bar<'a>(x: &'a i32) {
}

Lifetimes
You’ll also need explicit lifetimes when working with structs that contain
references.

Lifetimes
struct Foo<'a> {
x: &'a i32,
}
fn main() {
let y = &5; // this is the same as `let _y = 5; let y = &_y;`
let f = Foo { x: y };
println!("{}", f.x);
}

Lifetimes
We need to ensure that any reference to a Foo cannot outlive the reference to an
i32 it contains.

Thinking in scopes
fn main() {
let y = &5; // -+ y goes into scope
// |
// stuff // |
// |
} // -+ y goes out of scope

Thinking in scopes
struct Foo<'a> {
x: &'a i32,
}
fn main() {
let y = &5; // -+ y goes into scope
let f = Foo { x: y }; // -+ f goes into scope
// stuff // |
// |
} // -+ f and y go out of scope

Thinking in scopes
struct Foo<'a> {
x: &'a i32,
}
fn main() {
let x; // -+ x goes into scope
// |
{ // |
let y = &5; // ---+ y goes into scope
let f = Foo { x: y }; // ---+ f goes into scope
x = &f.x; // | | error here
} // ---+ f and y go out of scope
// |
println!("{}", x); // |
} // -+ x goes out of scope

Not Covered
Macros
Tests
Concurrency
Foreign Function Interface
Cargo
and much much more

Resources
The Rust Programming Language. Also known as “The Book”, The Rust Programming Language is the
most comprehensive resource for all topics related to Rust, and is the primary official document of the
language.
Rust by Example. A collection of self-contained Rust examples on a variety of topics, executable in-
browser.
Frequently asked questions.
The Rustonomicon. An entire book dedicated to explaining how to write unsafe Rust code. It is for
advanced Rust programmers.
rust-learning. A community-maintained collection of resources for learning Rust.

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